Add browser close friendly err (#687)

[inancgumus]

This PR does the following:

• Allows executing Browser.Close only once.
• Warns users with "a friendly message" when it's called multiple times.

Notes:

• Close() is thread-safe as only the same VU or testBrowser use it.

You can try the result with this script:

import { chromium } from 'k6/x/browser';

export default async function () {
    const browser = chromium.launch();
    const page = browser.newPage();

    await page
        .goto('https://test.k6.io/', { waitUntil: 'networkidle' })
        .finally(() => {
            page.close();
            browser.close();
        });

    await page
        .goto('https://test.k6.io/contacts.php', { waitUntil: 'networkidle' })
        .finally(() => {
            page.close();
            browser.close();
        });
}

It should print:

WARN[0002] Please call browser.close only once, and do not use the browser after calling close.
  category="Browser:Close" elapsed="0 ms" goroutine=84

[ka3de]

Thanks for working on these optimizations!
LGTM.

[ankur22]

LGTM